1,1′-bi-2-naphthol [CAS. No 602-09-05] [I] is one of the industrially important chemicals and is produced in large quantities all over the world.

1,1′-bi-2-naphthol occurs in two optically active forms because of its restricted rotation viz. (S)-(+1,1′-bi-2-naphthol [CAS No. 18531-99-2] and (R)-(+)-1,1′-bi-2-naphthol [CAS No. 18531-94-7].
(S)-(−)-1,1′-bi-2-naphthol and (R)-(+)-1,1′-bi-2-naphthol are generally obtained by resolution of racemic i.e. (RS)-1,1′-bi-2-naphthol, which is synthesized by oxidative coupling of 2-naphthol in presence of transition metal salts having variable valence such as FeCl3, CuSO4, CuCl2, VO++ complex, etc. (J. Org. Chem. 1989, 54, 1252)
Both the optically pure (S)-(+1,1′-bi-2-naphthol and (R)-(+)-1,1′-bi-2-naphthol have wide applications in synthetic chemistry and are used as building blocks for the synthesis/manufacture of many important chemicals including natural products, (Tetrahedron 1995, 51, 9353; Tetrahedron 2000, 56, 2325); and also as chiral auxiliaries in stoichiometric quantity as well as in catalytic amount in various asymmetric syntheses.
The optically pure (S)-(−)-1,1′-bi-2-naphthol and (R)-(+)-1,1′-bi-2-naphthol are used as auxiliary or converted to specific chiral ligands for use in various asymmetric reactions, such as, enantioselective reduction, in various catalytic asymmetric Diels-Alder reactions, ene reactions, asymmetric Michael additions, alkylations, oxidations, epoxidations and nitroaldol reactions etc. (Chem. Rev. 1998, 98, 2405-2494; Chem. Rev. 2007, 107, PR1-PR45).
Recently, it has also been demonstrated that optically active 1,1′-bi-2-naphthol can also be used for optical resolution of active pharmaceutical compounds such as omeprazole, lamivudine etc. (OPRD, 2009, 13, 450-455) and as a chiral shift reagent for the determination of the optical purity and absolute configuration of a wide range of chiral compounds (Chirality, 2009).
The synthesis of enantiomerically pure (R) or (S)-1,1′-bi-2-naphthol has been extensively studied with essentially two major approaches such as resolution (enzymatic and chemical) and, asymmetric synthesis.
I) Enzymatic Resolution:
Kazlaukas has reported cholesterol esterase catalyzed enantioselective hydrolysis of binaphthol esters. The reported method requires an additional step for preparation of binaphthol esters but unfortunately the said enzyme is not commercially available. (J. Am. Chem. Soc., 1989, 111, 4953)
Enantioselective trans-esterification reaction of rac-1-indanol with rac-1,1′-binaphthyl-2-2-dibutyrate in presence of enzymes such as procine pancreatic lipase, procine pancreatin and cholesterol esterase have been reported. This method has been demonstrated for mutual separation of rac-1-indanol and racemic (RS)-1,1′-bi-2-naphthol but overall yield for optically pure 1,1′-bi-2-naphthol is low and needs an additional step for separation. Hence, this could not be an industrial process for obtaining optically pure 1,1′-bi-2-naphthol (Tetrahedron Lett. 1993, 34, 6057).
Enantioselective monomethyl etherification of racemic (RS)-1,1′-bi-2-naphthol in presence of bovine serum albumin has demonstrated low enantiomeric excess. (J. Am. Chem. Soc., 1996, 118, 9990)
Lipase catalyzed resolution of (RS)-1,1′-bi-2-naphthol has also reported low enantiomeric excess. The reported method needs an additional step for preparation of binaphthol esters. (Tetrahedron: Asymmetry 2003, 14, 289; Tetrahedron Lett. 2006, 47, 4797)
It is evident from the above that in enzymatic resolution methods, more often than not, the separation is not economic and also enzymes are not available commercially. Further, overall cost for obtaining optically pure 1,1′-bi-2-naphthol through enzymatic resolution is very high. Although, commercially available enzymes such as lipases are used for enantioselective hydrolysis of 1,1′-bi-2-naphthol ester, enantiomeric excess is far from desirable.
Asymmetric oxidative coupling is scientifically interesting and demonstrated by using Camellia sinensis cell culture or horseradish peroxidase. However, enantiomeric excess is far from satisfactory (Tetrahedron Lett. 2002, 43, 8499; Tetrahedron Lett. 1997, 38, 5695)
II) Chemical Resolution:
Resolution of (RS)-1,1′-bi-2-naphthol via formation of diastereomeric inclusion complexes with various chiral hosts is very well documented. Some of these methods reportedly gave low enantiomeric excess. Furthermore, in many cases overall yield is low thus rendering most methods economically unfeasible.
More details about literature methods are discussed hereinafter.
Chiral amide derivatives of succinic acid and tartaric acid are used for resolution of (RS)-1,1′-bi-2-naphthol. In the said method, synthesis of chiral amide derivatives is tedious and requires POCl3. Furthermore to obtain optically pure 1,1′-bi-2-naphthol from complex needs an additional step of forming complex with aqueous NH2NH2 followed by decomposition in presence of dilute hydrochloric acid (J. Org. Chem. 1988, 53, 3607-3609). FIG. 1 gives the schematic representation of resolution of (RS)-1,1′-bi-2-naphthol via amide derivatives of tartaric acid.
Resolution of (RS)-1,1′-bi-2-naphthol by forming inclusion complex with chiral cinchonidium halides such as N-benzylcinchonidium and n-butyl cinchonidium bromide (Tetrahedron Lett. 1995, 36, 7991-7994; J. Org. Chem. 1994, 59, 5748-5751), chiral 1,2-diaminocyclohexane (EP 471498), chiral m-tolyl methyl sulfoxide (Tetrahedron Lett. 1984, 25, 4929-4932) are not at all cost effective processes at industrial scale.
Resolution of (RS)-1,1′-bi-2-naphthol by forming inclusion complex with L-proline (Tetrahedron: Asymmetry, 1995, 6, 341-344); (R)-(α)-methyl benzylamine (J. Org. Chem. 1999, 64, 7643-7645); (R)-2-aminobutanol (Synthesis 1990, 3, 222-223); (S)-5-oxopyrrolidine-2-carboxanilide (JP 08245460, Tetrahedron Lett. 2002, 43, 5273-5276) have demonstrated poor enantiomeric excess and in most of these cases, overall yield is very low. Hence, they cannot be used as an industrial process for obtaining optically pure 1,1′-bi-2-naphthol.
Resolution of (RS)-1,1′-bi-2-naphthol is also demonstrated by reaction with (R)-menthyl chloroformate (J. Org. Chem., 1995, 60, 6599-6601); neomenthylthioacetic acid chloride (Tetrahedron: Asymmetry 1995, 6, 111-114) and separation of diastereomers by crystallization. However, lithium aluminum hydride, a hazardous chemical, is used to decompose the complex, which is not operation friendly to handle at large scale. Hence it cannot be used as an industrial process for obtaining optically pure 1,1′-bi-2-naphthol. Moreover, cost per unit kg of final product is also high. FIG. 2 gives the schematic representation of resolution of (RS)-1,1′-bi-2-naphthol via (R)-menthyl chloroformate.
(III) Formation of Phosphate Ester Diastereomeric Inclusion Complexes:
Resolution of (RS)-1,1′-bi-2-naphthol is also demonstrated by forming phosphate ester of (RS)-1,1′-bi-2-naphthol followed by diastereomeric complex with optically pure α-methylbenzylamine (J. Org. Chem. 1995, 60, 7364-7365) or L-menthol (J. Org. Chem. 1993, 58, 7313-7314), which is further separated by crystallization. Above said methods use LiAlH4, which is not environmentally benign and require special handling conditions. Hence it cannot be used as an industrial process for obtaining optically pure 1,1′-bi-2-naphthol. FIG. 3. gives the schematic representation of resolution of (RS)-1,1′-bi-2-naphthol via phosphate ester diastereomeric complex with chiral α-methyl benzyl amine.IV) Formation of Borate Ester Diastereomeric Inclusion Complexes:Resolution of (RS)-1,1′-bi-2-naphthol is also demonstrated by forming borate ester of (RS)-1,1′-bi-2-naphthol and further treating with (R)-α-methylbenzylamine (J. Org. Chem. 1999, 64, 7643-7645); quinine (CN 1097728); TMEDA (Tetrahedron: Asymmetry 1996, 7, 2471-2474) or L-Proline to obtain diastereomeric complex (Tetrahedron: Asymmetry 1998, 9, 3985), which is subsequently separated by crystallization. Most of the above methods have demonstrated poor enantiomeric excess and overall yield is also very low. Hence these cannot be used as industrial processes for obtaining optically pure 1,1′-bi-2-naphthol. FIG. 4. represents the schematic representation of the resolution of (RS)-1,1′-bi-2-naphthol via borate ester complex.(V) Conglomerate Separation:Toda et al., reported the process for resolution of (RS)-1,1′-bi-2-naphthol through inclusion complex formation with racemic or achiral ammonium salts and transformation of complex into a conglomerate (Tetrahedron, 60, 2004; 7767-7774). WO 99/12623 describes the process for separation of (RS)-1,1′-bi-2-naphthol by forming inclusion complex with N-methylpyrrolidine and subsequent conglomerate separation.VI) Asymmetric Synthesis:US Patent 2005/0256345 describes the process for asymmetric oxidative coupling of 2-naphthol in presence of vanadium complex. Egami and Katsuki have reported iron catalyzed asymmetric aerobic oxidative coupling of 2-naphthol (J. Am. Chem. Soc. 2009, 131, 6082-6083). Rate of reaction of vanadium complex based asymmetric oxidation is very slow whereas iron catalyzed asymmetric coupling reports poor enantiomeric excess.U.S. Pat. No. 6,570,036 describes the co crystallisation process for separation of racemic naphthylethyl amine with (S)-ibuprofen and (S)-diacetone ketogulonic acid. However, there is no embodiment of single crystal data, Powder X-ray diffraction analysis and hydrogen bond interaction pattern for co-crystal.
It is evident from prior art that there is a need for an eco-friendly, “green”, cost effective, easy-to-operate industrial-scale synthesis of optically pure compound viz. (S)-(−)-1,1′-bi-2-naphthol and (R)-(+)-1,1′-bi-2-naphthol.
This invention provides that.